Method for alkaliating electrodes

ABSTRACT

The present invention relates to a method for lithiation of an intercalation-based anode or a non-reactive plating-capable foil or a reactive alloy capable anode, whereby utilization of said lithiated intercalation-based anode or a plating-capable foil or reactive alloy capable anode in a rechargeable battery or electrochemical cell results in an increased amount of lithium available for cycling, and an improved reversible capacity during charge and discharge.

RELATED APPLICATIONS

This application is a continuation of U.S. Application No.: 16/937,108,filed on Jul. 23, 2020, which is a continuation of U.S. Application No.:16/156,297, filed on Oct. 10, 2018 (abandoned), which is a continuationof U.S. Application No.: 14/590,573, filed on Jan. 6, 2015 (now U.S.Pat. No. 10,128,491), which is a continuation-in-part of U.S.Application No.: 13/688,912, filed on Nov. 29, 2012 (now U.S. Pat. No.9,598,789), which claims the benefit of U.S. Provisional Application No.61/662,115, filed on Jun. 20, 2012 and U.S. Provisional Application No.61/565,580, filed on Dec. 1, 2011. The entire teachings of the aboveapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

In the field of rechargeable batteries or electrochemical cells wheremetal ions are shuttled between cathode and anode at varying voltages,the initial source of metal ions (usually alkali metal) is typically thecathode material. An example of said metal ions includes lithium.

During the initial cycling of a lithium ion rechargeable battery,passivation films are formed on the anode and cathode, but particularlyon the negative electrode. As shown in FIG. 1 , several reactions cantake place as this film is formed on the negative electrode, includingsolvent reduction, salt reduction, insoluble product formation, andpolymerization. The passivation film is often referred to as an SEIlayer (solid electrolyte interphase), the formation of which results inthe loss of metal ions through an irreversible reaction, as well as asignificant loss in battery capacity. Most often, lithium ion batteriesare described as having an irreversible initial loss of 10 to 30%. Asecond type of irreversible loss of metal ions (e.g., lithium+) is dueto side reactions that occur during the “shuttling” of metal ions duringeach additional charge and discharge cycle of the metal ion battery. Athird type of irreversible loss is represented by a cathode passivationlayer formation composed of soluble and insoluble lithium salts.

Precautions are taken to limit all types of irreversible losses (SEI,cathode passivation layer, and side reactions during long cycling). Itwould be advantageous, however, if a source could be provided tocompensate for the excess metal ion requirement, in an amount necessaryto support long cycle life. In most commercial metal ion batterysystems, this reserve is provided by the cathode, and therefore thecathode must necessarily be sized to be about 135 to 150% of thespecified discharge capacity of the battery, thus increasing the totalweight of the battery. Once the irreversible loss of metal ions relatedto SEI and cathode passivation layer formation is complete, up to 30% ofthe cathode’s metal-donating material has become “dead weight”, ornon-operating material. Examples of these heavy and expensive cathodematerials are LiFePO₄, LiMn₂O₄, etc.

There have been attempts to source lithium metal to the anode during theconstruction of the anode. For example, FMC Corporation (Philadelphia,PA) has developed a stabilized lithium source called stabilized lithiummetal powder, or SLMP (U.S. Pat. No. 8,021,496). This material can bemixed into carbon before an activation step, such as crushing ordissolving by the electrolyte (U.S. Pat. No. 7,276,314). However, SLMPis a very expensive lithium source compared to even common cathodedonating materials, and may be difficult to distribute evenly. Dendritesmay be enabled by un-dissolved lithium particles causing catastrophicshorts to develop.

Another example of sourcing metal to the anode is found in Li/polymerbatteries, where Li metal is placed on a current collector to form ananode containing all the required overcapacity. The coulombic efficiencyof this approach, however, is low when compared to the graphite anodebased gel or liquid electrolyte battery approach. Furthermore, while thespecific capacity is the highest possible, the cost of lithium metalfoil is fairly high and the discharge rates for the necessary solidpolymer electrolytes are low.

Others have attempted to increase the amount of alkali metal that isavailable during charge/discharge of an electrochemical cell using aprocess called pre-lithiation, first charging, or pre-charging, whereina passivation film is either chemically or electrochemically formed onthe anode prior to final assembly of the battery (US 5595837; US5753388; US 5759715; US 5436093; and US 5721067). In the cases whereelectrochemical pre-lithiation was conducted, either alithium-containing electrode (most often consisting of elemental lithiummetal), or a lithium foil was employed as the source of lithium. Analternate process that circumvents the formation of a passivation film,and thus the need to use pre-lithiation, is disclosed in U.S. Pat. No.5,069,683.

SUMMARY OF THE INVENTION

The present invention relates to the discovery of an improved processfor lithiating (and/or alkaliating) a material, such as electrodes,specifically anodes or cathodes over known commercial processes. Thenovel processes overcome problems in the prior art by providing a goodelectrochemical process that is roll to roll compatible with currentassembly methods, can use inexpensive Li bearing salts with good toexcellent efficiency, particularly when used in combination withnon-aqueous solvents that do not react with the material, (e.g., anodeor cathode) during lithiation. Preferred non-aqueous solvents dissolveand do not ionize the lithium salts, match the desired electrochemicalwindow, or are substantially inert to the anode binder material.Preferred solvents possess a boiling point distant to that of water. Asolvent or solvent condition that would meet all of the criteriaincluding salt solubility, ionic conductivity, electrochemical window,and ease of water separation is preferred.

Although several lithium bearing salts can be used in electrolysis, onlythe least expensive such as LiCl, LiBr, LiF, and LiNO₃ for non-limitingexamples are preferred for low cost production. Until now, there hasbeen no process that would allow these types of salts to be used asfeedstock in the production of battery electrodes (anodes or cathodes).Until now, 30% efficiency would have been the limit using non-aqueoussolvents, making production too costly. A satisfactory refinementprocess has not been found to produce low moisture, pure solvent/saltsolutions. Compounds formed by side reactions will eventually interferewith the formation of a successful SEI layer; only insoluble SEImaterial is desirable (usually formed in the complete battery cell),while these typical electrolysis byproducts are not. Until now, acontinuous refinement process has not been found, making it impracticalto pre-lithiate battery or electrochemical cell anodes using salt as afeedstock. For the purpose of eliminating the mentioned limitations andcreating a low cost pre-lithiated anode, a novel process is nowdisclosed.

For the purpose of this discussion, lithiation is the electrochemicalintroduction of lithium into and/or on a material (preferably anelectrode, anode or a cathode) and includes: electrochemicallytransporting ions of lithium into the material as in intercalation;electrochemically transporting lithium ions onto, e.g., an anode currentcollector surface as in plating; electrochemically transporting lithiumions into an alloy of, e.g., an anode metal as in alloying; and/orelectrochemically transporting lithium ions into a surface layer of thematerial, e.g., an anode or cathode, e.g. an SEI layer. Plating refersto forming a layer of atoms onto the immediate surface of a substrate,usually a metal through an electrolytic process. Alloying refers to aplating process where lithium atoms wind up in a homogeneous mixturewithin the host substrate, such as with aluminum or tin. Intercalationrefers to a process where lithium ions are inserted between planes of ananode host material such as carbon or silicon or a cathode material suchas Nickel Magnesium Cobalt (NMC). In some instances within thedescription, lithiation, intercalation, lithium plating or alloying areused interchangeably. In each instance where lithiation with salt isdiscussed below, lithiation with a lithium halide salt is understood tobe preferred. Additionally, it is possible to use other alkali metalhalide salts or alkali metal salts in lieu of lithium salts to achievealkaliation in each of the processes described herein.

By lithiating the material, e.g., anode or cathode, prior to batteryassembly, a surplus of lithium is present that can support longercycling life, initial losses due to SEI formation, cathode relatedalkali metal ion losses, and/or alkali metal free cathode materialcycling needs. The lithiation method can be implemented on a continuousor batch basis. In one embodiment, a metal-intercalating material, suchas carbon, graphite, tin oxide, and silicon, is coated onto a currentcollector of a conductive material such as copper, coated aluminum orcarbon fiber, forming the intercalation-based anode. A bath containing anon-aqueous solvent such as, but not limited to gamma butyrolactone(GBL), and at least one dissolved lithium salt such as, but not limitedto LiCl, contacts the anode. Other solvents can be used and preferablyare selected to exhibit: adequate salt solubility; a suitableelectrochemical window; good ionic conductivity; low temperature boilingpoint under high vacuum conditions (e.g. less than 130° C. at 1 mTorr)to reduce risk of solvent degradation; a differential boiling point fromwater (e.g. 25° C. minimum) to facilitate water separation; miscibilitywith other cyclic and linear solvents; and/or no propensity to attacktypical anode binders. Other lithium salts can be used, preferably onesthat produce easily managed byproducts, more preferably those that havegaseous byproducts. Preferably the salt should also exhibit lowsolubility in common linear solvents such as DMC so that the salt may berecovered and cleansed easily for reuse after lithiation. A sparging gassuch as CO₂ or SO₂ can be added to the lithiation bath in order to:increase the lithium salt solubility; increase the ionic conductivity;improve the quality of the SEI layer; and/or increase the lithiationefficiency as has been discovered. Electrolytic field plates areprovided. A reducing current is applied to the anode in such a way as tolithiate. At the field plate, there is an oxidizing current, so there isa need to use an inert material such as platinum, or carbon. In anotherpreferred embodiment, the byproduct of the lithiation process is limitedto an evolving gas at the counter electrode (field plate). The fullcomplement of lithium ions is provided in this way. In anotherembodiment, a lithium non-interactive current collector may be platedusing this method. In another embodiment, an alloying metal foil orcoating may be lithiated in this method.

After lithiation, it may be desirable to reduce the amount of remainingsalt or lithiation solvent in the anode. An additional step comprising arinse with a solvent (such as GBL or DMC) but without the salt contentto reduce the remaining salt content in the lithiated anode can beperformed. In the case of cathode rinsing, the electrode may beelectrically held in reduction during the rinse to prevent oxidationwith chlorine. Alternately a pair of rollers can be used to removeexcess surface fluids from the anode as it departs the lithiation tank.Alternately, the processed and rinsed material, e.g., anode or cathode,can be vacuumed dried, thereby removing the remaining solvent, making itcapable of long-term storage and compatible with subsequent use in anormal battery assembly process.

The invention provides a method for lithiation of a material, e.g., ananode or cathode, preferably, in a continuous process, comprising thesteps of:

-   (a) providing a material, e.g., an anode or cathode;-   (b) providing a bath comprising a non-aqueous solvent and at least    one dissolved lithium salt, preferably a lithium halide salt, such    as lithium chloride, wherein said bath contacts the anode,    preferably in a continuous process, and wherein a dry gas blankets    said bath;-   (c) providing an electrolytic field plate comprising an inert    conductive material wherein said field plate establishes a field    between the anode and the field plate; and-   (d) applying a reducing current to the anode and an oxidizing    current to the field plate, wherein metal ions from the bath    lithiate the material, e.g., the anode or cathode.

The invention also discloses a method for lithiating in a continuousprocess, wherein the lithiated material provides for the reducedirreversible capacity for the whole cell or provides for the wholeamount of lithium necessary to operate a non lithium metal containingcathode material, comprising the steps of:

-   (a) providing a material, e.g. an anode or cathode, comprising a    lithium active material, or an inactive substrate that can be    plated;-   (b) providing a bath comprising a non-aqueous solvent and at least    one dissolved lithium halide salt, wherein said bath contacts the    material, e.g. anode or cathode, in a continuous process, and    wherein a dry gas blankets said non-aqueous solvent and at least one    dissolved lithium halide salt;-   (c) providing an electrolytic field plate, comprising an inert    conductive material wherein said field plate establishes a field    between the material, e.g. anode or cathode, and the field plate;-   (d) applying a reducing current to the material, e.g., an anode or    cathode, wherein metal ions will lithiate the material, e.g., anode    or cathode, in a continuous process;-   (e) applying an oxidizing current to the field plate; and-   (f) collecting an evolving gas or byproduct generated at the field    plate.

In one embodiment, the anode material is selected from carbon, coke,graphite, tin, tin oxide, silicon, silicon oxide, aluminum,lithium-active metals, alloying metal materials, and mixtures thereof,wherein said material is coated onto a current collector of a conductivematerial selected from copper or aluminum respectively or carbon fiber.

In a further embodiment, the non-aqueous solvent is selected frombutylene carbonate, propylene carbonate, ethylene carbonate, vinylenecarbonate, vinyl ethylene carbonate, dimethyl carbonate, diethylcarbonate, dipropyl carbonate, methyl ethyl carbonate, acetonitrile,gamma-butyrolactone, room temperature ionic liquids, and mixturesthereof. In a preferred embodiment, the non-aqueous solvent isgamma-butyrolactone.

In another embodiment, the halide salt is that of Na or K. In anotherembodiment, the lithium containing salt is LiNO₃. In yet anotherembodiment, the lithium halide salt is selected from LiCl, LiBr, LiF,and mixtures thereof. In a preferred embodiment, the lithium halide saltis LiCl.

In yet another embodiment, the non-aqueous solvent contains an additivethat facilitates the formation of a high quality SEI layer. For example,VC, EC or maleic anhydride could be added to the non-aqueous solvent.

In yet another embodiment, a sparged gas such as CO₂ or SO₂ isincorporated into the lithiation bath in order to: increase the saltsolubility, increase the ionic conductivity, support good quality SEI inthe form of Li₂CO₃ or Li₂SO₃, and increase the efficiency of theintercalation. The sparged gas is bubbled to create up to atmosphericpressure saturation. Higher levels of saturation are also beneficial,but this level of gas saturation is sufficient to increase theefficiency of lithiation. CO₂ is preferred because of its lower cost andlower toxicity. Samples of graphite anodes were pre-lithiated to 1mAhr/cm² in the described method but with and without bath CO₂saturation. The resulting anodes were delithiated against a lithiummetal counter electrode in a quartz beaker to +1 volt above lithiummetal to determine the reversible lithium content. In all cases, thereversible lithium amount was greater in the CO₂ examples than thosewithout CO₂. FIG. 9 shows the improvement of the lithiation process whenCO₂ is sparged into the lithiation process tank. This represents asignificant improvement to any commercial application of pre-lithiation.

In each instance where lithium, lithium salts and/or lithiation arediscussed below/above, it is understood that other alkali metals andalkali salts can be used and alkaliation can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 : Characterization of the SEI layer on carbon negative electrode(anode).

FIG. 2 : Lithiation tank layout.

FIG. 3 : Lithiation system layout with a solvent conditioning system andsalt replenishment system.

FIG. 4 : Multi-tank lithiation layout.

FIG. 5 : First charge and discharge of a standard anode versus LiFePO₄cathode.

FIG. 6 : First charge and discharge of a pre-lithiated anode versusLiFePO₄ cathode.

FIG. 7 : Lithiation system layout with a solvent conditioning system,solvent distillation system, and salt replenishment system.

FIG. 8 : Cell capacity comparison of LiCoO₂/graphite control versuspre-lithiated button cell.

FIG. 9 : Lithium intercalation efficiency with CO₂ and without CO₂.

FIG. 10 : Coulombic efficiency of pre-lithiated and control samplecells.

FIG. 11 : Comparative heat test results of pre-lithiated and controlsample cells.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Anodes comprised of metal oxides or metal alloys or graphite or carbonor silicon or silicon/carbon blends, such as anodes comprised ofgraphite or carbon, are lithiated during the first charging step in thebattery operation after assembly, with lithium coming from the cathodematerial. In these cases, the cathode is the heaviest and most expensivecomponent in the battery. It would therefore be desirable and ofcommercial importance to reduce the weight of the cathode, with minimalloss to the battery efficiency and output. If the dead weight thatresults from SEI and cathode passivation layer formation could beeliminated by sourcing the metal ions in such a way that alleviated theeffects of the irreversible losses of the metal ions, then the specificcapacity and volumetric capacity density of the battery could beincreased, and cost of the battery could be reduced. In some cases, itmay be beneficial to place an amount of lithium into the cathode that isslightly above the as produced stoichiometric value. The presentinvention relates to a method for lithiation of an intercalation-basedmaterial, such as an anode or cathode, a non-reactive plating-capablefoil, or an alloying capable film or foil whereby utilization of saidlithiated material e.g., an anode or cathode, in a rechargeable batteryor electrochemical cell results in an increased amount of lithiumavailable for cycling, and an improved reversible capacity during chargeand discharge. The additional lithium available may also support thecycling of an initially non-lithium-containing cathode material.Alternately, an initially non lithium containing cathode material suchas sulfur can be lithiated directly prior to assembly. As mentionedabove, anodes comprised of graphite or carbon or silicon orsilicon-carbon blends have been lithiated during the first charging stepin the battery operation after assembly, with lithium coming from thecathode material. In these cases, the cathode is the heaviest and mostexpensive component in the battery. One of the desired features inlithium battery technology is to reduce the weight of the battery comingfrom the excess cathode material, without compromising batteryefficiency and output.

A method for fabricating a lithiated material, e.g., an anode orcathode, which provides increased amounts of lithium available forcycling, improved reversible capacity during charge and discharge of arechargeable battery and a consequent lighter battery is disclosed inFIG. 2 . Electrolytic field plates are held at a voltage necessary toestablish a field between the anode or cathode and the field plate, andto lithiate the anode, such as to plate or intercalate lithium onto afoil, or into an anode or cathode substrate or sheet, or to form an SEIlayer upon the anode or cathode. A typical operating voltage for this is4.1 V. An appropriate reference electrode, such as Ag/AgNO₃ non-aqueousreference from Bioanalytical Systems, Inc., located close to thetargeted negative electrode may be preferred to monitor the anode orcathode conditions. It is possible to operate the field plates in eithervoltage or current control mode. With current control, the fulloperating potential may not be immediately obtained. This operationunder current control may result in lower initial operating voltages.This lower voltage may prefer secondary side reactions instead of thedissociation of the lithium halide salt (e.g. LiCl) and the resultingintercalation of the anode material. Operating under voltage control canensure that the field plate potential is immediately set to a sufficientpotential to favor the dissociation of the lithium halide salt (e.g. 4.1Volt for LiCl) and to minimize secondary side reactions. Current controlcan alternatively be used if the subsequent operating voltage remainsabove the lithium halide salt dissociation threshold. This can be doneby setting a sufficiently high initial current density (e.g. 2 mA/cm²)that will favor the dissociation rather than secondary side reactions.An oxidizing current is applied at the field plate, so there is a needto use an inert material or a conductive oxide. In one embodiment, theinert material comprising the field plate is selected from glassycarbon, tantalum, gold, platinum, silver, and rhodium. In a preferredembodiment, the inert material comprising the field plate is selectedfrom platinum, gold or carbon. In a more preferred embodiment, the inertmaterial comprising the field plate is carbon or glassy carbon. Thefield plates may also be comprised of a base material such as stainlesssteel that is plated with an inert conductive material such as gold,platinum, or glassy carbon. The field plates are immersed within thebath, with the anode passing between the field plates as illustrated inFIGS. 2 and 4 . The field plates can be operated as a single entity at asingle controlled voltage or current density, or multiple plates can beimplemented that allow for independent control of voltage or currentdensity over multiple zones. This is illustrated in FIGS. 2 and 4 .

The anode typically comprises a compatible anodic material which is anymaterial which functions as an anode in an electrolytic cell. As hereindisclosed, the term anode is equivalent to the terms negative electrode,conductive foil, anode sheet, anode substrate, or non-reactiveplating-capable foil. In one embodiment, anodes arelithium-intercalating anodes. Examples of materials that compriselithium-intercalating anodes include but are not limited to carbon,graphite, tin oxide, silicon, silicon oxide, polyvinylidene difluoride(PVDF) binder, and mixtures thereof. In a further embodiment,lithium-intercalating anode materials are selected from graphite, cokes,mesocarbons, carbon nanowires, carbon fibers, silicon nanoparticles orother metal nanomaterials and mixtures thereof. In another embodiment,alloying metals such as tin or aluminum may be used to host the lithiummetal as a result of the lithiation. A cathode is a substance typicallycoated on a current collector that gives up lithium ions and electronsduring the charging step of an electrochemical cell. Examples of thesecathode materials include but are not limited to LiFePO₄, LiMn₂O₄ etc. Areducing current is applied to the electrode in such a way as tointercalate the lithium. The anode or cathode is bathed in a solutioncomprising a non-aqueous solvent and at least one dissolved lithiumsalt. The term non-aqueous solvent is a low molecular weight organicsolvent added to an electrolyte which serves the purpose of solvatingthe inorganic ion salt. Typical examples of a non-aqueous solvents arebutylene carbonate, propylene carbonate, ethylene carbonate, vinylenecarbonate, vinyl ethylene carbonate, dimethyl carbonate, diethylcarbonate, dipropyl carbonate, methyl ethyl carbonate, acetonitrile,gamma-butyrolactone, triglyme, tetraglyme, dimethylsulfoxide, dioxolane,sulfolane, room temperature ionic liquids (RTIL) and mixtures thereof.In one embodiment, a non-aqueous solvent is selected from ethylenecarbonate, vinylene carbonate, vinyl ethylene carbonate,gamma-butyrolactone, and mixtures thereof. In a second embodiment, anon-aqueous solvent is gamma-butyrolactone. In a third embodiment, anadditive can be introduced to support high quality SEI formation. Theadditive could be vinylene carbonate, ethylene carbonate or maleicanhydride. In a fourth embodiment, a gas such as CO₂ or SO₂ is spargedinto the non-aqueous solution in order to: increase salt solubility;increase the ionic conductivity; support the formation of a Li₂CO₃ orLi₂SO₃ SEI layer; and increase the lithiation efficiency. FIG. 9describes the efficiency of reversible lithium intercalation from aninitial amount of lithium sourcing measured in mAhr. The lost amount canbe described as side reactions such as but not limited to SEI formation.

The term alkali metal salt refers to an inorganic salt which is suitablefor use in a non-aqueous solvent. Examples of suitable alkali metalcations comprising an alkali metal salt are those selected from Li⁺,Na⁺, K⁺, Rb⁺, Cs⁺, Fr⁺, and mixtures thereof. Examples of suitablehalogen anions comprising an alkali metal salt are those selected fromF⁻, Cl⁻, Br⁻, I⁻, and mixtures thereof. In one embodiment, the alkalimetal salt is selected from LiF, LiCl, LiBr, NaF, NaCl, NaBr, KF, KCl,KBr, and mixtures thereof. Other salts such as LiNO₃ may be used, but inthe preferred embodiment, the alkali metal salt is the halide LiCl.

Inexpensive salts with gaseous decomposition products can be halidessuch as LiCl, LiBr, and LiF. LiCl and other simple salts can bedifficult to dissolve or ionize in non-aqueous solvents. Solvents suchas propylene carbonate (PC), dimethyl carbonate (DMC), and acetonitrilesupport only trace amounts of LiCl in solution without the use of acomplexing agent such as AlCl₃. AlCl₃ and other complexing agents can bedifficult to handle in regard to moisture management and highcorrosivity. In addition, some solvents that can dissolve halide salts,such DMSO or tetrahydrofuran (THF), do not allow complete ionization ofthe salt, and/or attack the binding polymers in the anode composites.Gamma-butyrolactone has been found to facilitate the dissolution andionization of the desirable alkali metal halide salts. It combines goodsolubility of the alkali metal halide salts with compatibility with TFETeflon_(c), PVDF, butadiene rubber and other binders. The use of halidesalts with gaseous decomposition products such as LiCl prevents theproduction of solid precipitates during the lithiation process. Sincethe lithiation process products are primarily lithium ions and gas,there are few solid precipitates or intermediate compounds that canaccumulate in the non-aqueous solvent solution. Removal of dissolved gasfrom the non-aqueous solvent solution is preferred over solidprecipitates during long term continuous operation of a productionsystem.

Gamma-butyrolactone also has a capable electrochemical window, includingthe lithium potential near —3 volts vs. a standard hydrogen electrode(SHE). It is a capable electrolyte with high permittivity and lowfreezing point, and can dissolve and ionize up to a 1 M concentration ofLiCl. A modest amount of heat can be used to reach this value. In oneembodiment, the heat to dissolve and ionize up to a 1 M concentration ofLiCl is between about 30° C. and 65° C. In a more preferred embodiment,the heat is between about 38° C. and 55° C. In a most preferredembodiment, the heat is about 45° C. The lithiation tank can also havean internal circulating pump and distribution manifold to preventlocalized salt concentration deprivation.

It has been discovered here that a dissolved gas such as CO₂ or SO₂ canenhance the lithiation process. It increases the solubility of the salt,the ionic conductivity of the non-aqueous solvent, and doubles theefficiency of lithiation. Since CO₂ is inexpensive, easily dried,chemically safe, and a potential building block gas for a high qualitySEI layer, it has been selected as the preferred dissolved gas. CO₂preferentially reacts with trace H₂O and Li⁺ during the lithiationprocess to form a stable, insoluble SEI material (Li₂O, Li₂CO₃ etc.).FIGS. 8 and 10 exemplify the operating efficiency of the LiCoO₂/graphitecells with and without pre-lithiation. The moisture level in thelithiation tank is driven down by the consumption of CO₂ and H₂Oaccording to this process, and care is given to control the moisturelevel in the tank to between about 5 to 20 ppm (see FIG. 2 ). In thisway, anode lithiation with a quality SEI material is producedcontinuously.

The intercalation or plating process for lithium ions (or generallylithiation) from 1 M LiCl salt in gamma-butyrolactone solvent will occurat about 4.1 volts measured between the anode sheet and the referenceelectrode up to a reducing current density of 2 mA/cm² or more. Asintercalation rates are increased too far beyond this current density,dendrites or lithium plating may begin to take place which harm thefinal battery or electrochemical cell performance. This will varydepending on the graphite porosity etc. In order to control both thecurrents and dependant voltages accurately, it may be necessary todivide the field plate into zones as shown in the FIGS. 2 and 4 . Othermetals can also be plated or intercalated with this method includingsodium as an example. As mentioned above, the byproduct of theintercalation process when using a halide alkali metal salt is anevolving gas at the counter electrode (field plate). In a preferredembodiment, the evolving gas is selected from F₂, Cl₂, Br₂, and mixturesthereof. In a more preferred embodiment, the evolving gas is Cl₂.

Prior to entering the lithiation bath, the anode or cathode material canbe pre-soaked in an electrolyte solution as shown in FIG. 4 . Thepre-soaking of the material will ensure full wetting of the materialprior to the start of the lithiation process. This pre-soak bath cancontain a non-aqueous solvent with or without a lithium salt, with orwithout a sparge gas, and with or without an SEI promoting additive.

The evolution of gas at the field plate or counter electrode can resultin evolving gas entering into, and/or being released from, the bathsolution. As a result, controlling the buildup of dissolved and releasedgas is desired to avoid corrosion, as for example, in the hypotheticalcase of trace water contamination reacting with chlorine gas, to formHCl during chlorine gas evolution. The tank assembly can be configuredto control the introduction of moisture into the system by using a drygas blanket on top of the liquid. In one embodiment, the dry gas (1-10ppm moisture) is selected from helium (He), neon (Ne), argon (Ar),krypton (Kr), xenon (Xe), sulfur hexafluoride (SF₆), nitrogen (N₂), dryair, carbon dioxide (CO₂) and mixtures thereof. In a preferredembodiment, the dry gas is selected from nitrogen, argon, carbondioxide, dry air and mixtures thereof. Moisture ingress can also becontrolled by having a long narrow gap entry and exit tunnel for theanode film where a counter flowing dry gas is used to mitigate air entryinto the system.

FIGS. 2, 3, 4 and 7 illustrate a process and apparatus that continuouslycontrols moisture, gas, and small quantities of lithiated organiccompounds during a continuous lithiation process. Liquid is drawn from abath through a series of valves. The liquid can be delivered in a batchmode to a refluxing unit, or it can be continuously circulated through aconditioning loop including distillation or reverse osmosis. The refluxunit can take batches of material through a vacuum refluxing processthat will remove both accumulated gas as well as moisture from theliquid. In one embodiment, the accumulated gas is selected from F₂, Cl₂,Br₂, and mixtures thereof. In a more preferred embodiment, theaccumulated gas is Cl₂. The use of reflux conditioning instead of adistillation process can prevent a change in the salt concentration ofthe working fluid which would result in a loss of salt content throughprecipitation. Once the batch liquid has been refluxed for a designatedperiod of time, the liquid can be returned to the bath with a lowermoisture and gas content. The size and rate of the reflux unit can bematched to the moisture ingress rate and to the gas production rate inorder keep the bath liquid at optimum conditions. The reflux rate can beincreased through use of multiple simultaneous batches and through theuse of high rate reflux equipment such as a rotary evaporator and highvacuum conditions. The reflux batch moisture content typically decays inan exponential fashion and the turnover rate can be tuned for optimalmoisture control with minimal energy input and equipment cost.

The refluxing unit can be placed after a salt dosing unit. The saltdosing unit can be used to add and mix the desired salt into thenon-aqueous solvent solution. The temperature of the dosing unit can beheld to maximize the solubility of the salt in the electrolyte and theelevated temperature can also be used as a pre-heating step for therefluxing unit. In one embodiment, the dosing unit maintains an elevatedprocess temperature of between about 30° C. and 65° C. In a morepreferred embodiment, the dosing unit maintains an elevated processtemperature of between about 38° C. and 55° C. In a most preferredembodiment, the dosing unit maintains an elevated process temperature ofabout 45° C. The benefit of dosing in the salt in a dosing unit beforethe refluxing unit is that the salt does not have to be in a completelydry state. Removing the moisture from a solid phase salt can be verydifficult. Once a salt is dissolved into solution, however, the watercontent of the salt can be removed through the refluxing process.Maintaining the dosing unit at an elevated temperature increases thesolubility of the lithium salt in the non-aqueous solvent and ensuresfull dissolution of the salt prior to the refluxing unit.

The conditioning/replenishment loop operates in a continuous mode andcan also be used to remove dissolved gases from the bath liquid throughuse of a membrane contactor. The gas output from the membrane contactorand the reflux unit can be passed through a scrubber to capture anyeffluent, such as chlorine gas, produced by the process. In oneembodiment, the dissolved gases are selected from F₂, Cl₂, Br₂, andmixtures thereof. In a more preferred embodiment, the dissolved gas isCl₂. The bath liquid can also be paired against either vacuum or a drygas within the membrane contactor in order to remove unwanted gases. Inone embodiment, the dry gas is selected from helium (He), neon (Ne),argon (Ar), krypton (Kr), xenon (Xe), sulfur hexafluoride (SF₆) nitrogen(N₂), carbon dioxide (CO₂), dry air and mixtures thereof. In a preferredembodiment, the dry gas is selected from nitrogen, argon, carbondioxide, dry air and mixtures thereof.

An inline heater can be used to maintain an elevated tank temperature tomaintain consistent bath operating conditions, even with variations infacility temperature. Elevated lithiation tank temperatures can aid inthe formation of a high quality SEI layer. In one embodiment, the inlineheater maintains an elevated tank temperature of between about 30° C.and 55° C. In a more preferred embodiment, the inline heater maintainsan elevated tank temperature of between about 30° C. and 45° C. In amost preferred embodiment, the inline heater maintains an elevated tanktemperature of about 40° C.

A filter unit can be used to remove any accumulated particulatecontamination. The filter unit can be located at various points in theloop including prior to the pump and after the salt dosing unit. Thefilter unit can be used to remove particulates from the non-aqueoussolvent in cases where a non-halide lithium salt such as LiNO₃ is usedsuch that a precipitate is formed at the field plates.

Lithium halide salt can be added to the non-aqueous solvent using thesalt dosing unit. An excess of solid lithium salt can be maintainedwithin the dosing unit to keep the lithium salt concentration within theloop and within the bath at the desired level (i.e., a saturatedsolution of about 0.5 M to 1.0 M) over long periods of time. The dosingunit can be configured to keep the solid salt from entering the bath orrefluxing unit. By dosing salt prior to the refluxing unit, there is noneed to separately dry the salt with its high water binding energy inits granular state. In one embodiment, the lithium salt within the saltdosing unit is selected from LiF, LiCl, LiBr, and mixtures thereof. In apreferred embodiment, the lithium halide salt within the salt dosingunit is LiCl. Dissolved lithium salts can be carried through the rest ofthe loop. The fluid circulation loop pump rate can be matched tomaintain a constant lithium salt concentration in the tank. For a givenanode or cathode substrate process rate, a matching loop circulationrate will dose the same amount of lithium salt as the lithiation processconsumes. As the anode or cathode process rate is increased ordecreased, the loop circulation rate can be modified to maintain anequilibrium state within the bath.

Depending on the specific tank conditions, the bath fluid can be treatedusing a circulating loop, a refluxing unit or a distillation unit asshown in FIGS. 2 and 4 . A circulating loop can dose in salt, removedissolved gases, control the bath temperature and removed particulatecontaminants. A refluxing unit is effective at removing dissolved gasesand for removing moisture content without reducing the salt content ofthe solution. A distillation unit is effective at removing dissolvedgases, removing moisture content, removing all salt content and removinglithiated organic compounds. The output from the distillation unit canbe fed back into a dosing and refluxing unit to reestablish the saltcontent if required. The effluent from the distillation unit can becollected and treated to recover used salt for reuse in the lithiationprocess. For example, DMC solvent will rinse away all but the insolublesalt so that the salt may be re-introduced into the dosing unit.Recirculating loops, refluxing unit and distillation units can be sharedacross multiple tanks that have different input and output requirementsas a means of minimizing equipment size and cost.

When the anode is lithiated to the extent of the irreversible andextended cyclic loss amount, it can be assembled into a rechargeablebattery or electrochemical cell with a smaller amount of lithium-bearingcathode material than would otherwise be required, thereby improving thespecific capacity, specific energy, volumetric capacity density andvolumetric energy density of the cell while reducing its cost.Alternately, a cathode can be pre-lithiated to or above the normalstoichiometric value to supply excess lithium to the forming cell.

When the anode is lithiated to the extent of the irreversible andextended cyclic loss amount, as well as the intended cycling amount, itcan be assembled into a battery or electrochemical cell with a cathodematerial that does not initially contain lithium. This type of cathodematerial can be much less expensive than lithium containing cathodematerials, and examples include, but are not limited to, MnO₂, V₂O₅ andpolyaniline. Alternatively, the cathode itself may be pre-lithiatedprior to assembly. The cost of the battery or cell produced with thismethod will be lower due to the lower cost of the feedstock lithiumsalt.

Therefore, previous limitations to the low cost production of moreefficient lithium ion batteries and electrochemical cells are overcomeby the novel processes described here. The materials and processes ofthe present invention will be better understood in connection with thefollowing examples, which are intended as an illustration only and notlimiting of the scope of the invention.

EXAMPLES

The following is a detailed example of an anode preparation andprocessing. 25 micron thick copper foil was cleaned with isopropylalcohol and Kimberly-Clark Kimwipes to remove oil and debris and thendried in air. A solution was prepared by adding 2.1 grams of 1,000,000weight PVDF powder from Arkema Fluoropolymers Div. to 95 ml of dry NMPsolvent from Aldrich Chemical. The solution was mixed with a stir barovernight to fully dissolve the PVDF material. The solution was kept inthe dark to prevent the light sensitive solvent from reacting. 33.9 mlof this PVDF solution was then added to 15 grams of Conoco PhilipsCPreme G5 graphite and 0.33 grams of acetylene black and stirred for 2hours in a ball mill at 600 RPM without any mixing balls. The resultingslurry was cast onto the copper foil using a vacuum hold down plate withheating capability. The finished graphite thickness after casting anddrying at 120° C. was about 100 microns or 14 mg/cm₂. The anode sheetwas then die punched into 15 mm diameter discs and then pressed at about3000 psi and 120° C. for use in a 2032 button cell assembly. Thecopper/graphite anode discs were then vacuum baked at 125° C. and about1 mTorr in a National Appliance Company model 5851 vacuum oven for atleast 12 hours.

The anode discs were then transferred into a Terra Universal dry airglove box with -65° C. dew point air supplied by compressed dry airpassed through a Kaeser two stage regenerative drier. The anode discswere then vacuum infiltrated with a GBL solvent with a 0.5 Mconcentration of LiCl salt solution. This electrolyte solution had beenprepared by heating to 90° C. and then vacuum refluxing at about 1 mTorrfor 6 hours to remove moisture down to about 10 ppm. The anode discswere allowed to soak for a half hour at vacuum conditions, a half hourin atmospheric pressure conditions and a half hour in the lithiationvessel itself prior to any currents being passed. The lithiation vesselincluded a constant bubbling of CO₂ gas to achieve a saturation leveland a temperature of 30° C. Test leads from a Maccor 4300 battery testerwere connected to the anode sample (red working) and glassy carbon(black counter) electrode. Voltage at the working electrode is monitoredvia a Ag/AgNO₃ non-aqueous electrode. A reducing current of 2 mA/cm² wasapplied to the graphite anode until a total of 1 mAhr/cm² was achieved.The pre-lithiated anode disc was then rinsed in pure distilled GBL andvacuum dried. The anode discs were then assembled against either LiFePO₄or LiCoO2 12 mm diameter cathode discs. The separator used was Celguard2400, and about 0.2 ml of electrolyte was used in the assembly. Theelectrolyte was 1:1:1 EC:DMC:DEC with 1M LiPF₆ salt and 1% VC withmoisture levels at about 10 ppm. A vacuum was applied to the assembledcell to remove bubbles before crimping in an MTI model MT-160D crimpingtool. Subsequent electrical tests were then performed on the batterytester unit using a 12 hour delay, two about C/12 formation cycles to atleast 3.7 volts, about C/3 charge/discharge cycles, and 20 minute reststeps between them. All the battery tests were carried out in ahome-made environmental chamber controlled to 26° C.

A Maccor model 4300 battery tester was used to test the CR2032 sizebutton cells assembled with a CPreme graphite anodes, LiFePO₄ or LiCoO₂cathodes, and Celguard 2400 separators. Electrolyte solutions containinga 1:1:1 mixture of EC:DMC:DEC with 1 molar concentration of LiPF6 saltand 1% VC were used. Both anodes and cathodes were cast with PVDFbinders. First charge and discharge cycles, often called the formationcycles, were performed at a current rate of approximately C/12. FIGS. 5and 6 illustrate the first cycle irreversible loss using pre-lithiatedand non-pre-lithiated graphite anodes mounted against LiFePO₄ cathodes.The initial absolute charge capacity of the two samples is different dueto extraneous packaging variation. The irreversible losses arerepresentative of the methods described, however. In FIG. 5 , thereversible capacity of the button cell is 56%. In FIG. 6 , thereversible capacity of the button cell when matched to a pre-lithiatedanode is 98%. FIG. 8 shows a typical LiCoO₂/graphite vs. aLiCoO₂/pre-lithiated graphite, but otherwise identical sample, testedover an extended range of charges and discharge cycles at approximatelya C/3 rate. The results indicate that there is a long lasting benefit tothe battery cell due to pre-lithiation using the methods described. FIG.11 shows the effectiveness of the SEI layer formed during theprelithiation process, by comparing capacity retention of the cell withprelithiated anode to a control cell, with both cells being subjected to48 hours of 50° C. heat as a form of accelerated aging test.

Example 2

An NMC cathode is mounted in a half cell containing lithium metal as thenegative electrode. The NMC cathode is “charged” to liberate most of itslithium content. This cell is disassembled and the cathode taken out touse as an electrode in the pre-lithiation apparatus that includes theabove mentioned GBL and LiCL salt solution including CO2 gas. The NMCcathode is subjected to a reducing current of 1 mA/cm2 with a totaldosage of 1 mAhr/cm2. The cathode is then rinsed while held in a smallreducing current (0.1 mA/cm2) to inhibit chemical oxidation and thenvacuum dried. The cathode is then mounted into a half cell as describedearlier and cycled starting with the “charge” step to measure the amountof cycleable lithium that is present. The amount of cycleable lithiumcan be measured. After the charge step is completed, normal cycling canbe performed at a rate of C/3 between 4.2 volts and 3.0 volts. The halfcell capacity can be determined.

An example of a salt other than LiCl that has been used by the inventorto achieve lithiation is LiNO₃. Reasonable rates of lithiation have beenachieved with LiNO₃. When the anodes pre-lithiated using LiNO₃ werepaired with LiFePO₄ cathodes, however, poor cycling resulted, possiblydue to an unidentified byproduct. This problem can be solved by aslightly more complicated removal process such as an additional anoderinse.

While there has been illustrated and described what is at presentconsidered to be the preferred embodiment of the present invention, itwill be understood by those skilled in the art that various changes andmodifications may be made and equivalents may be substituted forelements thereof without departing from the true scope of the invention.Therefore, it is intended that this invention not be limited to theparticular embodiment disclosed, but that the invention will include allembodiments falling within the scope of the appended claims.

1. A method for lithiation of a material, preferably, in a continuousprocess, comprising the steps of: (a) providing a material; (b)providing a bath comprising a non-aqueous solvent and at least onedissolved lithium halide salt, wherein said bath contacts the material,preferably in a continuous process, and wherein a dry gas blanket coverssaid bath; (c) providing an electrolytic field plate comprising an inertconductive material wherein said field plate establishes a field betweenthe material and the field plate; and (d) applying a reducing current tothe material and an oxidizing current to the field plate, whereinlithium ions from the bath lithiate into the material.
 2. The method ofclaim 1, wherein the material is an anode active material selected fromcarbon, coke, graphite, tin, tin oxide, silicon, silicon oxide,aluminum, lithium active metals, alloying metal materials, compositesand mixtures thereof.
 3. The method of claim 1, wherein the non-aqueoussolvent is selected from butylene carbonate, propylene carbonatevinylene carbonate, vinyl ethylene carbonate, dimethyl carbonate,diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate,acetonitrile, gamma -butyrolactone, room temperature ionic liquids, andmixtures thereof.
 4. The method of claim 3, wherein the non-aqueoussolvent is gamma -butyrolactone.
 5. The method of claim 1, wherein thehalogen of the dissolved lithium halide salt is selected from ionic F⁻,Cl⁻, Br⁻, I⁻ and mixtures thereof.
 6. The method of claim 1, wherein thedissolved lithium halide salt is LiCl.
 7. The method of claim 1, whereinthe dissolved lithium halide salt is LiBr.
 8. The method of claim 1,wherein the dissolved lithium halide salt is LiF.
 9. The method of claim1, wherein the electrolytic field plate is selected from platinum, gold,glassy carbon, and graphite.
 10. The method of claim 1, wherein thelithiated material and foil are used in the final assembly of arechargeable battery.
 11. The method of claim 1, wherein the lithiatedmaterial is used in the assembly of an electrochemical cell to providethe lithium needed for cycling when paired with a cathode not initiallycontaining lithium.
 12. The method of claim 1, comprising the step ofperforming a pre-charging cycle upon the anode externally prior to theassembly of an electrochemical cell.
 13. The method of claim 1, whereinthe evolving gas generated at the field plate is captured by a refluxunit, a membrane contactor, a gas scrubber, and combinations thereof.14. The method of claim 1, comprising one or more reflux units, membranecontactors, gas scrubbers, baths, inline heaters, filters, salt dosingunits, pumps, valves, and combinations thereof, connected in a loopcomprising series and parallel connections.
 15. The method of claim 14,wherein said inline heaters heat a non-aqueous solvent and dissolvedalkali metal halide salt to a temperature of between 30° C. and 65° C.16. The method of claim 15, wherein said temperature is about 40° C. 17.The method as in claim 1, wherein a separate immersion bath is used torinse the material in a solvent while holding the electrode in areducing current mode.
 18. A method as in claim 1, wherein the salt isrecovered periodically by distillation of the used non-aqueous solventand subsequent rinsing of the salt in a light non -solvating fluid. 19.The method of claim 14, wherein the rate of said continuous process canbe increased and decreased.
 20. The method of claim 17, wherein the rateof continuous lithiation of the anode and foil can be increased anddecreased.
 21. The method of claim 17, wherein the rate of loopcirculation can be increased and decreased.
 22. The method of claim 3,wherein the non-aqueous solvent contains an additive to facilitate highquality SEI formation.
 23. The method of claim 22, wherein the additiveis vinylene carbonate.
 24. The method of claim 1, wherein a dissolvedgas is added.
 25. The method of claim 24, in which the dissolved gas iscarbon dioxide.
 26. A method for alkaliation of a material, preferably,in a continuous process, comprising the steps of: (a) providing amaterial; (b) providing a bath comprising a non-aqueous solvent,dissolved CO2 or SO2 gas and at least one dissolved alkali metal salt,wherein said bath contacts the material, preferably in a continuousprocess, and wherein a dry gas blanket covers said bath; (c) providingan electrolytic field plate comprising an inert conductive materialwherein said field plate establishes a field between the material andthe field plate; and (d) applying a reducing current to the material andan oxidizing current to the field plate, wherein metal ions from thebath alkaliate into the material.